1. Field
The present invention relates generally to systems and methods for irradiating targets with electromagnetic radiation, and more specifically to systems having arrays of dipole antennas and associated control system for controlling application of electromagnetic radiation to targets through phased array power steering.
2. State of the Art
The use of electromagnetic (EM) energy to heat tissue for the treatment of disease is well known. Electromagnetic energy applicators, such as microwave energy antennas, can be arranged with respect to the tissue to be treated to apply electromagnetic energy to the tissue to be treated to heat such tissue to desired treatment temperatures. Generally the tissue to be treated is diseased tissue, such as a tumor, located within normal healthy tissue which needs to be preserved and not treated. In such heat treatments, it is important to ensure that adequate heat is developed in the tissue to be treated without overheating surrounding healthy tissue. Various systems are currently available for applying electromagnetic energy to tissue to be treated to heat such tissue.
Some systems for applying electromagnetic energy to tissue to be treated located within normal tissue not to be treated control the position of the region of heating within the tissue through phased array power steering. In such systems, a plurality of electromagnetic applicators are arranged in an array around the tissue to be treated. Each applicator is separately powered by a separate channel of a multi-channel EM power system so different applicators are each provided with electronically controlled power of electronically controlled phase by respective separate power channels of the EM power system. This creates a desired phased array heating pattern steering capability. By controlling the relative power level and phase of the EM signal provided by each of the applicators to the tissue, the size, configuration, and location of the heating region can be controlled so as to provide adequate heat to the tissue to be treated while minimizing the heating of the normal healthy tissue not to be treated. The BSD-2000 system produced by BSD Medical Corporation, Salt Lake City, Utah, is a multi-channel phased array system that controls frequency, radiated power, and relative phase for each of a plurality of applicators. Each channel provides electronic control of power and phase and is connected to a different applicator. This allows electronic steering and shaping of the heating region.
In phased array heating systems, a plurality of applicators positioned around the tissue to be treated apply EM signals to the tissue from different directions so that the signals interact, such as by constructive interference, to create a heating zone in the tissue. In the use of phased array heating systems, the control of the relative power levels and phase of each applicator of the plurality of applicators is important in order to provide a desired heating region and heating pattern to adequately heat the tissue to be heated to provide the desired treatment while minimizing the heating of the normal healthy tissue surrounding the tissue being treated. There are many factors affecting the characteristics of the various EM signals in the tissue and the interactions between the signals. This makes it difficult to accurately control the positioning of and the heating pattern of the heating zone and to reduce the possibility of hot spots outside the heating zone.
Pretreatment planning using modeling has become an important aspect of providing heat treatment to body tissue. In general, pretreatment planning can be used to plan the treatment to be administered to a patient. While various degrees of pretreatment planning are used, the most comprehensive pretreatment planning usually involves the use of a computer program to simulate the heating patterns predicted by the program to be produced by particular applicators placed in particular locations or patterns in and/or around the tissue to be treated and operated at particular operating parameters such as particular frequencies, particular phases, particular power levels, etc. These simulation programs provide for the designation by a user of the location, size, and shape of the tissue mass to be treated and the position of the tissue mass in relation to the applicators. The user can then select particular operating parameters such as particular frequencies, particular phases, particular power levels, etc. for particular applicators and the simulation program produces a predicted heating distribution pattern. These simulated predicted heating distribution patterns are compared to the location, size, and shape of the tissue to be treated to determine how good the match is between the simulated heating pattern and the location, size, and shape of the actual tissue to be treated. The goal is to ensure that during the actual treatment adequate heat is developed in the diseased tissue to be treated without overheating surrounding healthy tissue. If a particular simulated predicted heating distribution pattern does not correspond well to the size and shape of the tissue to be treated, the user or the simulation program can make changes in the number of applicators used, their locations, the properties of the applicators, and/or the applicator operating parameters to try to obtain a better match.
The end result of the pretreatment planning simulation is a representation of a predicted heating distribution pattern that has been chosen as predicting the closest match to the desired treatment, i.e., the distribution pattern as predicted by the simulation program to provide the best treatment of the diseased tissue. As indicated above, a predicted simulated heating distribution pattern is produced taking into account specific positioning of the one or more applicators, specific applicator characteristics, and specific applicator operating parameters. Therefore, the predicted simulated heating distribution pattern chosen to produce the best tissue treatment indicates the applicator positioning, applicator characteristics, and applicator operating parameters predicted to provide the chosen heating distribution pattern. In setting up for the actual treatment, the treatment system is set up to operate at the same operating parameters used to obtain the chosen predicted simulated heating distribution pattern. With this set up and operation, it is predicted that the actual treatment result in the patient will be the treatment as shown by the chosen predicted simulated heating distribution pattern. Thus, the chosen simulated heating distribution pattern obtained in the pretreatment simulation provides the user with the optimized applicator positioning, applicator characteristics, and applicator operating parameters for the user to use during actual treatment of the patient.
However, the actual results obtained during treatment may be different from the predicted results so it is important to monitor the actual treatment to ensure that it proceeds as planned and expected. Various factors contribute to inaccuracies in the simulated treatment, such as the complexity of the treatment system and the accuracy of the model used to predict the treatment. In phased array heating systems, the plurality of applicators apply EM signals to the tissue from different directions so that the signals interact, such as by constructive interference, to create the heating zone in the tissue. There are many factors affecting the characteristics of the various EM signals in the tissue and the interactions between the signals. Further, because a phased array system has a plurality of antennas, there is cross coupling between various antennas that can affect the signals produced. This makes it difficult to accurately predict the positioning of the heating zone and the heating distribution in the heating zone and to predict locations of hot spots both inside and outside the heating zone.
The BSD-2000 phased array hyperthermia system uses an array of dipole antennas in a ring that surrounds the patient's body, with a water filled bolus interface between the system antennas and the patient's body. The antennas used are dipole couplets, i.e., two parallel side-by-side dipole antennas which are separated by and have their feedpoints connected by a coupling tee transmission line. In one model of the BSD-2000, four such dipole couplets (eight microwave dipole antennas) are arranged around the patient with each couplet attached to a separate channel of the BSD 2000 system. The currently used arrangement of four dipole couplets is a Sigma 60 applicator. In another model of the BSD-2000, twelve such dipole couplets (twenty four microwave dipole antennas) are arranged around the patient in three longitudinally spaced rings of four couplets each, with each couplet attached to a separate channel of the BSD 2000 system. The currently used arrangement of twelve dipole couplets is a Sigma Eye applicator. The use of numerical modeling to predict the specific absorption rate (SAR) of the EM power applied to the tissue and the resulting heating distribution in the tissue in patients treated using the BSD-2000 phased array hyperthermia system has been an ongoing research and development project since the 1980's. Such a numerical model could be used not only for pretreatment planning but also for interactive treatment control during treatment. Such development efforts begun at BSD Medical Corporation in the 1980's and continued at the University of Utah, Dartmouth, and Stanford University, have resulted in the creation by Nadobny and Seebass of Berlin of what is now known as the SigmaHyperPlan patient specific pretreatment planning program.
The SigmaHyperPlan is a numerical predictive program that uses a patient specific dielectric model generated from CT or MR scans of the tissue to be treated and the tissue around the tissue to be treated in the patient to calculate the SAR distribution within the patient for various frequency, relative power, and phase steering conditions for the array. The SigmaHyperPlan is used to predict the heating patterns that will be created within specific patients based on specific system operating parameters prior to treatment with the BSD-2000 and includes optimization of steering to maximize the predicted heating of target tumors. The BSD-2000 currently controls the forward power and phase and monitors the forward power and phase as well as reflected power for each of the channels driving RF power to each of the dipole antenna couplets of the Sigma applicators at various operating frequencies of the systems. With control of these variables and the monitoring of these variables for each of the antenna couplets, various assumptions have been made in the SigmaHyperPlan program in order to predict the SAR and the temperature distribution in the patient. The SigmaHyperPlan was developed based on the assumption that the feedpoint EM-fields, i.e., the EM field at the feedpoint of each dipole antenna, is directly based on the forward power and phase of the forward power driven to that couplet. In modeling each of the dipole antennas, it is further assumed that each of the dipole antennas in a dipole couplet have the same feedpoint E-field power and phase. Therefore, the SigmaHyperPlan program for modeling the Sigma Eye applicator can have an input of 12 forward powers and 12 forward power phases. The SigmaHyperPlan program for modeling the Sigma 60 applicator can have an input of four forward powers and four forward power phases. The numerical methods used in the SigmaHyperPlan are either finite difference time domain (FDTD) or finite element (FE) methods.
In pretreatment planning use of the SigmaHyperPlan program, the patient specific dielectric model generated from CT or MR scans of the tissue to be treated and the tissue around the tissue to be treated is used in conjunction with selected feedpoint e-fields for the respective antennas of the antenna phased array to be used for treatment. The initial selected feedpoint E-fields can be provided by a user of the pretreatment program or the program can start with a default setting of the respective antenna feedpoint E-fields. The program runs a simulation to determine the predicted heating pattern based on the selected antenna E-fields. If that program does not provide the desired heating pattern desired to treat the tissue to be treated, the program changes the values of the selected E-fields in an iterative process until it finds the best predicted heating pattern to provide the desired treatment. The program then provides the operator with the forward power and forward power phase settings to use for each channel to provide the feedpoint E-field for each antenna based on the presumption that each antenna feedpoint E-field is produced directly by the value of input power and input power phase applied to the antenna.
It has been found that improvement in the SAR and heating distribution predictions provided by the SigmaHyperPlan program can be obtained by improving the accuracy of the feedpoint E field information used in modeling the dipole antennas for each dipole antenna couplet. As indicated above, the two dipole antennas in each dipole couplet are separated by and have their feedpoints connected by a coupling tee transmission line. Also, a tuning stub is usually included in this coupling tee transmission line. Rather than assuming that the feedpoint E-field power and phase for each of the dipole antennas of a dipole couplet are the same and are directly based on the forward power and phase of the power driven to that couplet, it has been found that the coupling tee transmission line affects the values of the forward power and phase driven to the antenna feedpoints and that cross coupling between various antennas can result in the feedpoint E-Fields not being the same as had been predicted based on forward power and forward power phase. Adjustments for this can be included in the SigmaHyperPlan model as explained in Nadobny J, Fahling H, Hagmann M J, Turner P F, Wlodarczyk W, Gellermann J M, Deuflhard P, Wust P., Experimental and numerical investigation of feed-point parameters in a 3-D hyperthermia applicator using different FDTD models of feed networks. IEEE Trans Biomed Eng 2002; 49(11):1348-1359. However, the described adjustments for this are directed mostly to the effects of the coupling tee transmission line and are very limited in consideration of cross coupling between channels.
While the SigmaHyperPlan, and other similar programs for use with phased array hyperthermia systems provide guidance in planning and in operation of the phased array systems, such programs are still not very accurate in predicting the SAR and heating patterns during actual use of the system. Careful monitoring of tissue heating during actual treatment is still necessary to ensure proper heat treatment of the diseased tissue and limited heating of the normal tissue.